Biofuels production from bio-derived carboxylic-acid esters

ABSTRACT

A process for producing biofuels compounds directly from carboxylic acid esters recovered from a fermentation system is described. The process involves taking a fermentation broth that has been reduced to a dry powder containing free organic acids; reacting the carboxylic acid in the powder with an alcohol solvent under a CO 2 -containing atmosphere in substantial absence of any other acid catalyst at a reaction temperature and pressure that corresponds to supercritical, critical or near critical conditions for at least one of the alcohol or CO 2  to synthesize an ester, then subjecting the ester to either hydrogenolysis or hydrogenation to form a biofuel.

PRIORITY CLAIM

The present Application claims benefit of priority from U.S. patentapplication Ser. No. 14/650,475, which is a national stage entry ofInternational Application No. PCT/US2013/073793, filed Dec. 9, 2013,which itself claims benefit of priority from U.S. Provisional PatentApplication No. 61/739,790, filed Dec. 20, 2012, the contents of whichare incorporated herein.

FIELD OF INVENTION

The present invention relates to a process for the production of certainbiofuels. In particular, the invention pertains to a method forproducing biofuel products from bio-derived esters of carboxylic acidsby means of either hydrogenation or hydrogenolysis.

BACKGROUND

Development of alternative fuels has gained increasing importance withincreased awareness of environmental concerns and in the wake ofeconomic pressures and fuel costs that have resulted from supplyvolatility of petroleum and other fossil-based resources. Policy makershave recognized this importance and have encouraged production andincreased use of biofuels. As interest has grown in moving away frompetrochemical or natural gas-derived hydrocarbon sources, some haveconcentrated on finding renewable and sustainable “green” carbonresources. Early so-called first generation biofuels are made byfermenting food products (e.g., starch-containing and sugary plants,such as corn, wheat, or sugar cane) into short-chain alcohols (e.g.ethanol, butanol) or transesterifying fatty acids liberated from plantoils by glycerolysis (e.g., rape seed, soybeans) to make fatty acidmethyl esters used as biodiesel. (See e.g., Cesar B. Granda et al.,“Sustainable Liquid Biofuels and Their Environmental Impact,”ENVIRONMENTAL PROGRESS, Vol. 26, No. 3, pp. 233-250 (October 2007),incorporated herein by reference.)

Carboxylic acids such as used to make biodiesel can also be made byfermentation from carbohydrate sources, in addition to being derivedfrom plant oils. Although the fermentative production of carboxylicacids, such as malic or succinic acid, has several advantages overpetrochemical-based fuels, the production costs for the bio-basedcarboxylic acids have been too high for bio-based production to becost-competitive with petrochemical production regimes. (See e.g., JamesMcKinlay et al., “Prospects for a Bio-based Succinate Industry,” APPL.MICROBIOL. BIOTECHNOL., (2007) 76:727-740; incorporated herein byreference.) In fermentation of starches and sugars, carboxylic acids aregenerated using microorganisms. Microorganisms require certainconditions for their functions. For example, with most commerciallyviable succinate producing microorganisms described in the literature,one needs to neutralize the fermentation broth to maintain anappropriate pH for maximum growth, conversion and productivity.

Currently, the carboxylic acids are recovered from fermentation brothsas salts instead of as free acids. Typically, the pH of the fermentationbroth is maintained at or near a pH of 7 by introduction of ammoniumhydroxide or other base into the broth, thereby converting thecarboxylic acid into the corresponding acid salt. Generation ofcarboxylic acid salts versus the free organic acid form brings aboutsignificant processing costs. About 60% of the total production costsare generated by downstream processing, because of the difficultiesassociated with the separation and purification of the acids and theirsalts in the fermentation broth.

Recovery of carboxylic acids as salts has a number of associatedproblems and requires several different steps in post-fermentation,downstream processing to isolate free acids and to prepare thecarboxylic acids for chemical transformation and to convert the rawacids to useful compounds. When salts are generated in conventionalfermentation processes, an equivalent of base is required for everyequivalent of acid to neutralize. The amount of reagent used canincrease costs. Further, one needs to remove the counter ions of thesalts so as to yield free acids, and one needs to remove and dispose ofany resulting waste and by-products. For instance, calcium salts of C₄diacids have a very low solubility in aqueous broth solutions (typicallyless than 3 g/liter at room temperature), and are not suitable for manyapplications for which a free acid species is needed, such as chemicalconversion to derivative products like butanediol and biofuels.Therefore, the calcium salt is typically dissolved in sulfuric acid,forming insoluble calcium sulfate, which can readily be separated fromthe free diacid. Calcium sulfate is a product having few commercialapplications, and accordingly is typically discarded as a solid waste inlandfills or other solid waste disposal sites. All of these individualoperational units contribute to the overall costs of the process.

To reduce waste and costs associated with generating free carboxylicacids and to improve the recovery yield, a need exists for a better,more direct method of recovering a variety of carboxylic acids, such asmalic or succinic acid, and which can provide a successful route tocombine a biologically-derived hydrocarbon feedstock with the productionof various biofuel products, such as ethane, propane, propanol orbutanol, by means of hydrogenation or hydrogenolysis. Such astreamlined, green process would be a welcome innovation.

SUMMARY OF THE INVENTION

The present invention concerns a process for producing biofuels. Theprocess involves: a) obtaining a fermentation broth containing at leastone free carboxylic acid, or a mixture of carboxylic acids, or a mixtureof carboxylic acids of at least one carboxylic acid and associatedalkali or alkaline earth metal salts thereof; b) drying saidfermentation broth containing free carboxylic acids into a powder; andc) reacting said carboxylic acids in said powder with an alcohol solventor mixture of at least one alcohol under a CO₂ atmosphere in substantialabsence of any other acid catalyst at a reaction temperature andpressure that corresponds to supercritical, critical or near criticalconditions for the alcohol or CO₂ to synthesize the corresponding esteror esters; d) subjecting the esters formed to either hydrogenolysis orhydrogenation to form one or more biofuels; and e) recovering the one ormore biofuels. The ester can be a mono-ester, di-ester, or tri-ester.Preferably, the ester is a di-ester or tri-ester. Monoesters can convertto mono-alcohols, diesters to diols, and triesters to triols. Thebiofuel products can be either an alkane or alcohol.

The esterification reaction temperature is between about 150° C. andabout 250° C., and the operational reaction pressure is between about400 psi and about 3,000 psi. Depending on the desired results, theesterification reaction can be run for at least about 4 hours.

Additional features and advantages of the present methods will bedisclosed in the following detailed description. It is understood thatboth the foregoing summary and the following detailed description andexamples are merely representative of the invention, and are intended toprovide an overview for understanding the invention as claimed.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic diagram illustrating an iteration of the presentprocess for esterifying an organic carboxylic acid derived fromfermentation broth, and further downstream processes that can isolatethe resulting esters and/or generate other compounds from such esters.

FIG. 2 is a schematic diagram showing an example of ester productionusing succinic acid derived from fermentation, and a downstream processin which Na and Mg salts are recycled back into the fermentationreactor, in accordance with a part of an embodiment of the presentprocess.

FIG. 3 is a diagram that illustrates CO₂-assisted esterification of freesuccinic acid in various alcohols that are converted to correspondingdimethyl, diethyl, or dibutyl esters, according to the presentinvention.

FIG. 4 is a diagram that illustrates CO₂-assisted esterification ofother polycarboxylic acids.

FIG. 5 shows a series of reaction diagrams that summarize variations intemperature for CO₂-assisted esterification of free succinic acidderived from fermentation broth.

FIG. 6 shows a series of reaction diagrams that summarize variations ininitial operational pressure for CO₂-assisted esterification of freecarboxylic acid according to the invention.

FIG. 7 shows a series of reaction diagrams that summarize variations intemperature, and reaction times for CO₂ assisted esterification of freecarboxylic acid according to the invention.

FIG. 8 shows a series of reaction diagrams that summarize the results ofreaction succinic acids and their Mg²⁺ and Ca²⁺ salts.

DETAILED DESCRIPTION OF THE INVENTION Section I.—Description A

The present disclosure describes, in part, a process for making variousbiofuel products by means of either hydrogenation or hydrogenolysis froma biologically-derived carbon source, such as sugar or other plant-basedcarbohydrates. The process joins an ability to recover a carboxylic acidfrom a fermentation broth, with an ability to use the acid as afeedstock for biofuel generating reactions in a streamlined procedure.The present process includes a method of converting the carboxylic acidto its corresponding ester (e.g., mono-ester, di-ester, or tri-ester) ina relatively efficient and cost effective manner.

As used herein the term “biofuels” refers to a gaseous, liquid, or solidsubstance that is used as a fuel, which is produced from renewablebiological resources such as plant, cellulosic, or agricultural biomassor derivatives thereof. In particular, a biofuel refers to a materialthat can be used in or as a transportation fuel in internal combustionengines, to power certain machinery, or energy generation applications.For instance, propanol and butanol can be a gasoline additive much thesame as ethanol. Butane and propane in liquefied petroleum gas (LPG) andethane in natural gas can be adapted as fuels in certain transportationsystems. Other biologically-derived hydrocarbons, like octanol/octane,or alkanes heavier than C₅ or C₆ may also be biofuels.

In particular, the present process involves obtaining a fermentationbroth, from which cell mass and insoluble compounds have been eitherremoved or not, containing a mixture of free carboxylic acid ofinterest, optionally with associated alkali or alkaline earth metalsalts (e.g., sodium, potassium, or magnesium salts); drying the raw orclarified fermentation broth containing free carboxylic acid into apowder; reacting the carboxylic acid in the powder with an alcohol undera CO₂-containing atmosphere in substantial absence of any other acidcatalyst at a reaction temperature and pressure corresponding tosupercritical, critical or near critical conditions for the alcoholand/or CO₂ to synthesize an ester; and subjecting the ester to eitherhydrogenation or hydrogenolysis to form a biofuel product. As usedherein the term “free carboxylic acid” refers to a carboxylic acidcompound that is at least 50% in its protonated state when in solution,at or below its pKa value. As used herein, the term “substantialabsence” refers to a condition in which an acid catalyst is eitherlargely or completely absent, or is present in de minimis or traceamount of less than catalytic efficacy. In other words, no other acidcatalyst is present, or is present at a level less than 10%, 5%, 3%, or1% weight/weight relative to the carboxylic acid in the reaction.

In another aspect, the present invention involves the discovery of asimple but effective way of producing esters from carboxylic acids thatare otherwise costly and difficult to isolate. An advantage of thepresent invention is that one can use free carboxylic acids directlyfrom a fermentation broth and generate corresponding esters therefromwithout the need to isolate or purify the acids from the fermentationbroth, as is necessary in conventional extractions from broth.

In comparison to certain fermentation processes that neutralize orconvert the organic acids to their salts, the present process providesan easier way to isolate and extract organic acids from a fermentationbroth. The present process eliminates a need for titration andneutralization of the fermentation broth that can precipitate metalsalts, and certain purification steps to produce a stock platformchemical. The free organic acids are converted into esters, which aresimpler to process and extract by distillation or other purificationtechniques without the use of expensive and complicated chromatographicseparation columns or resins. For instance in a conventional process,one would need to use ion exchange chromatography to isolate the acids.A small amount of salts may unavoidably carry-over after the ionexchange. Hence, one may require multiple units of operation to purifythe acid to an acceptable quality level. With each added operationalunit the costs of the overall process increases. In contrast with thepresent process in synthesizing the ester of the acid, one can recoverthe salt as a carbonate or hydroxide, which can be used to regeneratethe fermentation broth, and minimize waste. Rather, an advantage of thepresent process is that one may further recycle the synthesisby-products directly back into the fermentation broth. By converting thecarboxylic acids to their corresponding esters, we can avoid suchissues.

B

Conventionally, esters are produced when carboxylic acids are heatedwith alcohols in the presence of an acid catalyst. The mechanism for theformation of an ester from an acid and an alcohol are the reverse of thesteps for the acid-catalyzed hydrolysis of an ester. The reaction can goin either direction depending on the conditions used. In a typicalesterification process, a carboxylic acid does not react with an alcoholunless a strong acid is used as a catalyst. The catalyst is usuallyconcentrated sulfuric acid or hydrogen chloride. Protonation makes thecarbonyl group more electrophilic and enables it to react with thealcohol, which is a weak nucleophile.

In general terms, the present esterification method involves a reactionof fermentation-derived, free organic carboxylic acid with an alcohol ina carbon dioxide (CO₂)-containing atmosphere in substantial absence ofany other acid catalyst to produce esters. The esterification reactionis performed in solution under conditions that are either atsupercritical, critical or near critical temperatures and/or pressuresfor either the alcohol and/or CO₂. Under such conditions, we believethat CO₂ self-generates or functions in situ as an acid catalyst, andregenerates back after the esterification reaction is completed. It isbelieved that a reactive intermediate (monoalkylcarbonic acid) is beingmade in situ in large enough quantities to drive esterification andaffect ester production. This intermediate, having a similar pKa (e.g.,˜4-5) as the free carboxylic acid, functions as a carbonic acid, whichis much weaker than the usual strong acids. The observed trend ofgreater ester conversion at higher temperatures adduces a relativelylarge energy of activation for this process.

An advantageous feature of the inventive process is that activation ofthe free carboxylic acid as an acyl halide (e.g., fluoride, chloride,bromide) or by using strong mineral acids is unnecessary unlike withsome other techniques. Acyl halides are inconvenient to use becausethese species are inherently reactive, have issues with stability, wastetreatment, and can be cumbersome and costly to make.

In the present process, carbon dioxide functioning as a catalyst insteadof the usual strong acids removes the need to introduce a strong acidinto the esterification reaction. This feature can circumvent the usualneed to adjust pH values in order to remove the catalyzing acid,enabling a simpler and cleaner synthesis. One can simply proceed tofilter the resultant product to remove alkali or alkaline earth metalcarbonate or other salts. A cleaner product will save costs inpurification and downstream processing for conversion to other chemicalfeedstock.

The process described herein is a more environmentally benign way ofproducing esters. As it is believed that the carbon dioxide canself-generate an acid catalyst in situ in the presence of the alcoholduring the esterification reaction, the present method does not requirethe use or addition of another acid catalyst species. In other words,the reaction kinetics with CO₂ alone can drive the esterification in thesubstantial absence of any other acid catalyst. To reiterate, thepresent process does not require activation of free acids as, forexample, an acyl chloride or by strong acids (i.e., Fischeresterification).

In general, the esterification is conducted at an operational orreaction temperature between about 150° C. to about 250° C., at areaction pressure of between about 400 psi and 2,500 psi, for anextended period, such as 4 hours, up to about 12 hours. Typically, thetemperature can be in a range between about 170° C. or 190° C. to about230° C. or 245° C. (e.g., 175° C., 187° C., 195° C. or 215° C.), and theoperational pressure is between about 900 psi or 950 psi and about 2,200psi or 2,400 psi (e.g., 960 psi, 980 psi, 1020 psi or 1050 psi).Alternatively, the temperature can be in a range between about 180° C.to about 245° C. (e.g., about 185° C. or 200° C. or 210° C. to about220° C. or 235° C. or 240° C.), and the operational pressure is betweenabout 1000 psi and 2,350 psi (e.g., 1,100 psi, 1,200 psi, 1,550 psi,1,750 psi, 1,810 psi, or 1,900 psi). Other temperatures may be within arange, for example, from about 160° C. or 185° C. to about 210° C. or225° C., and other operational pressures may be within a range, forexample, from about 1,150 psi or 1,500 psi to about 1,800 psi or 2,000psi.

These reaction temperatures and pressures correspond to supercritical,critical or near critical conditions for the alcohol(s) or CO₂. Table 1lists, for purpose of illustration, critical parameters for some commonsolvents (i.e., methanol, ethanol, 1-propanol, 1-butanol, water, andCO₂).

TABLE 1 Critical Data for Select Substances (Yaws, C. L., ChemicalProperties Handbook. In McGraw-Hill: 1999; pp 1-29.) Substance NameMolecular Weight Critical Temp. (K)/° C. Critical Pressure (bar)/psiCritical Density (g/cm³) Methanol 32.042 512.58/239.43  80.96/1174.22550.2720 Ethanol 46.069 516.25/243.10 63.84/925.9209 0.2760 1-Propanol60.095  537.4/264.25 51.02/739.9839 0.2754 1-Butanol 74.122 563.0 ±0.3/289.85 45.0 ± 4.0/652.671 0.3710 Water 18.015 647.13/373.98220.55/3198.8071 0.3220 Carbon dioxide 44.010 304.19/31.04  73.82/1070.6685 0.4682At conditions above the critical point (i.e., critical temperatureand/or pressure), the fluid exists in a super critical phase where itexhibits properties that are in between those of a liquid and a gas.More specifically, supercritical fluids have a liquid-like density andgas-like transport properties (i.e., diffusivity and viscosity). Thiscan be seen in Table 2, wherein the typical values of these propertiesare compared between the three fluid types—conventional liquids,supercritical fluids, and gases.

TABLE 2 Comparison of Typical Physical Property Values of Liquids,Supercritical Fluids, and Gases. Property Liquid SCF Gas Density (g/mL)1  0.3 10⁻³ Diffusivity (cm2/s) 5 × 10⁻⁶ 10⁻³  0.1 Viscosity (Pa · s)10⁻³ 10⁻⁴ 10⁻⁵Likewise, “near critical” refers to the conditions at which either thetemperature or pressure of at least the alcohol species or CO₂ gas isbelow but within 150K (e.g., within 50-100K), or 220 psi (e.g., within30-150 psi) of their respective critical points. It is believed that astemperatures and pressures reach near critical, critical orsupercritical conditions, the solubility of the reagents are enhanced,which promotes the esterification reaction. In other words, the CO₂ gas,alcohol, and acid species are better able to interact under nearcritical, critical or supercritical conditions than under less rigorousconditions. The reaction does not require that both the alcohol speciesand CO₂ gas be at near-critical, critical or supercritical conditions;rather, the reaction is operative as long as either one of the speciessatisfies such a condition.

If the present esterification reactions are operated at highertemperatures and greater pressures, up to about 250° C. and 2,500 psi or3,000 psi, respectively, for reaction times of up to about 10 or 12hours, one can produce significant amounts of ester product atrelatively high selectivity and level of purity within a shorterreaction time than before, which was about 18-20 hours. At loweroperational temperatures (<190° C.), formation of monoester molecules ofpolycarboxylic acids is more prevalent, while reactions at temperatures≥190° C. or 195° C., will convert preferentially the polycarboxylicacids to diesters. By selecting a temperature in a higher range fromabout 190° C. or 195° C. or 200° C. to about 245° C. or 250° C., one canpreferentially drive the reaction to a greater rate of diesterconversion. The esterification can yield a minimum of about 50%,desirably at least 65% or 70%, of a diester of the carboxylic acid.Reactions that are performed at or near supercritical operatingconditions tend to produce better results. When operated at or nearcritical conditions of about 230° C. or about 240° C. for methanol andabout 31° C./1000 psi for CO₂, one is able to achieve conversions ratesof about 90% or better, typically about 93% or 95%. One can achieve highyields by adjusting the permutations of different combinations oftemperature and reaction times (e.g., higher temperatures and shorterreaction times (e.g., less than 10 or 12 hours, between 4 and 8 hours)or vice versa), which can be an advantage over current approaches. Withoptimization, esterification conducted at 250° C. under either the sameor greater CO₂ pressure, the yield would be nearly quantitative (i.e.,≥95% yield), for example, up to about 98%, 99%, or 99.9% conversion.

As the accompanying Examples will show, variation in reaction conditionssuggests that one can generate more diester product with highertemperatures and/or protracted reaction times. As stated before,however, different permutations in temperature can influence theduration of the esterification reactions to produce the same amount ofester product. The reactions according to the present method are notconducive to a significant degree of side product formation; hence onecan avoid cyclization of the carboxylic acids and other startingreagents. Potential dangers of decarboxylization at high temperatures(i.e., >145° C. or >150° C.) are not observed in the present method.

Using an amount of the alcohol solvent in excess of the carboxylic acid,one can produce a very clean esterification. The present synthesisprocess produces very clean ester products at about 70%-72% initialpurity, without generation of significant amounts of side products suchas low molecular weight acids—acetic or formic acid—molecularrearrangements or cyclic products, which one could normally find instandard acid catalyzed esterification at high temperatures. The esterscan be refined to achieve about 90-98% purity. The purification can beaccomplished, for instance, by means of crystallization, chromatography,or distillation.

Typically, the resulting ester products can be either monoesters ordiesters, or form a mixture of both. One can control the reaction todrive the esterification toward either one ester form or another. Forinstance, one may select an operational temperature and pressure thatpreferentially drives the esterification reaction towards formation ofdiester molecules. Likewise, one can control whether esters are formedfrom either a single carboxylic acid species (e.g., succinic acid) or amixture of multiple different kinds carboxylic acids (e.g., acetic,citric, lactic, malic, maleic, succinic acids) that may be present andderivable from fermentation broth. In other words, one can use a varietyof different kinds of carboxylic acids in accord with the presentesterification reaction to produce a variety of different esters. Theseesters, in turn, can be isolated, further modified in downstreamchemical processes and converted, in certain embodiments, into usefulcompounds such as for pharmaceutical, cosmetic, food, feed, polymermaterials or biofuels. For instance, succinic esters can be convertedinto a polymer, such as polybutylene succinate (PBS).

In the present esterification process, both the catalyst (CO₂) and theesterification reagent (alcohol) are present in large excess relative tothe amount of free carboxylic acid. CO₂ should be in the gas phaseduring the reaction phase, regardless of its origin (e.g., gas tank ordry ice), as the reaction is conducted at high temperatures. Addition ofsolid CO₂ is strategic in the case where sealed pressure reactors areused, in that it allows for slow sublimation of gaseous CO₂ formation asthe reaction apparatus is being assembled. This can minimize CO₂ loss.In a CO₂ (i.e., CO₂-containing) atmosphere, the concentration of CO₂ inthe reaction atmosphere can be at least 10% or 15% by volume, favorablyabout 25% or 30%, preferably greater than 50%. For better reactionresults, the concentration of CO₂ should be maximized. Desirableconcentrations of CO₂ are from about 75% or 80% to about 99.9% byvolume, typically between about 85% and about 98%. Nitrogen (N₂) gas orair is permissible in the reactor, but preferably the concentration ofgases other than CO₂ is kept at either a minor percentage (<50%) or deminimis amount.

Any liquid alcohol with an R-group of C₁-C₂₀ can serve as the solventreagent or first alcohol species. The R-group can be saturated,unsaturated, or aromatic. A mixture of different kinds of alcohols(e.g., C₁-C₁₂) can also be used in the reaction, but will produce acorresponding mixture of different esters depending on the particularR-group. Certain lower alcohol species with C₁-C₆ alkyl groups arepreferred as the reagent in the first esterification with CO₂ in view oftheir common availability, inexpensiveness, and mechanistic simplicityin the esterification reaction. Further, alcohols such as methanol,ethanol, propanol, or butanol are preferred because of parameters suchas their comparatively simple structure and that the reactions are moreeasily controlled with respect to the supercritical, critical or nearcritical temperatures and pressures of these alcohol species.Alternatively, in some embodiments, the alcohol can also be aC₂-C₆-diol. Esterification with a diol can generate monomers or lowmolecular weight oligomers that can be readily polymerized.

One can use a variety of different carboxylic acids, for example,selected from the group consisting of: a) monocarboxylic acids: formicacid, acetic acid, propionic acid, lactic acid, butyric acid, isobutyricacid, valeric acid, hexanoic acid, heptanoic acid, decanoic acid, lauricacid, myristic acid, and C₁₅-C₁₈ fatty acids; b) dicarboxylic acids:fumaric acid, itaconic acid, malic acid, succinic acid, maleic acid,malonic acid, glutaric acid, glucaric acid, oxalic acid, adipic acid,pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioicacid, glutaconic acid, ortho-phthalic acid, isophthalic acid,terephthalic acid; or c) tricarboxylic acids: citric acid, isocitricacid, aconitic acid, tricarballylic acid, and trimesic acid. Thecarboxylic acids can include a mix of associated alkali or alkalineearth metal (e.g., sodium, potassium, or magnesium) salts of thesecarboxylic acids. Desirably, the carboxylic acid is a polycarboxylicacid, such as a dicarboxylic or tricarboxylic acid.

The process can reduce the amount of waste by means of recycling ofby-products back into the fermentation broth, either in a continuous orbatch process. We have also found that in the present esterificationprocess, when free carboxylic acid is reacted with an alcohol and CO₂absent any other acid catalyst, the free protonated form of thecarboxylic acids has greater solubility in the alcohol solvent thantheir corresponding salts. Performed under similar reaction conditions,the esterification reaction using the free carboxylic acid as a reagentwill yield about 2-3 times greater amount of ester product than thereaction that uses the salt species as a reagent. (This result can beseen when one compares the reaction of accompanying FIG. 3B (free acid)with that of FIG. 5A (acid salt), and in Table 4, Examples 2 and 3 (acidsalt), with Examples 5 and 6 (free acid), respectively.) It is believedthat solubility is a factor for the difference. For instance, since thesolubility of magnesium salts in methanol and ethanol are significantlybetter than that of calcium salts, product yield from a reaction of acalcium salt is much lower than that produced from a starting reagent ofa corresponding magnesium salt.

The present invention includes a method for esterifying a polycarboxylicacid. The esterification method involves: providing and reacting asolution of one or more free carboxylic acids with an alcohol in CO₂atmosphere without the presence of any other acid catalyst; andselecting an operational reaction temperature or reaction pressurecorresponding to supercritical, critical or near critical conditions forthe alcohol and/or CO₂ to yield an ester. The reaction temperature andpressure conditions preferentially drive the reaction towards theformation of at least diester molecules over monoester molecules whenthe carboxylic acid is a polycarboxylic acid. As with the recoveryprocess, the operational reaction temperature is between about 150° C.and about 250° C., and the operational reaction pressure is betweenabout 400 psi and about 2,500 psi. Depending on the desired results, thereaction can be run for up to about 12 hours.

C

The esterification process described above can be integrated intofermentation-based production of carbon chain feedstocks and to providea more convenient method of generating esters from carboxylic acidsderived from a renewable source. The present invention provides a directroute by which biologically-derived carboxylic acids can be recovered ina simple, cost-efficient process from a fermentation broth, convertedinto esters, and then subjected to hydrogenation or hydrogenolysis tomake biofuels. The accompanying figures illustrate how the esters areintegrated into further downstream processes.

FIG. 1 is a schematic representation showing a general process ofextracting carboxylic acids from fermentation broth that includes aversion of the present esterification reaction integrated with furtherprocesses that can utilize the resulting esters. As shown, fermentationbroth 10 from a reactor is filtered (ultra) 12 to remove biomaterialssuch as cell mass, and yield carboxylic acids including their salts, byproducts and other compounds. All of these materials are then dried 13to make an unrefined mixture 14. This dried mixture of materials is thenreacted 16 in a liquid system with an alcohol (R—OH; R=alkyl C₁-C₁₂) andCO₂ at an elevated operational reaction temperature and pressure toyield either monoesters or diesters, or a mixture of both. Only thecarboxylic acids react in solution. The resulting mixture 18 is filtered20 to separate the esters 22 and other by-products 24. The esters aresoluble while other by-product compounds are insoluble. The by-productsinclude carbonate salts of calcium, magnesium, or sodium, which can berecovered and recycled 26 back into a fermentation broth 28 in thereactor 10. This recycling can lead to significant cost savings andimproves the efficiency of the overall fermentation and extractionprocess. The esters can be processed subsequently either by distillation30, hydrogenation 32, or hydrogenloysis treatment 34, respectively, toseparate the different esters, produce C₄ platform compounds andbiofuels (e.g., ethane, ethanol, butane, butanol, etc.).

Through the distillation process one can concentrate the esters bydriving off the alcohol, and then filter the by-products resultant fromester synthesis. Further distillation of a mixed-acid ester productmixture according to the boiling points of the different ester species,permits one to separate the various individual esters. For instance,Table 3 provides boiling points for a sample of common esters that maybe present in an ester product mixture according to the presentinvention.

TABLE 3 Boiling Points for Some Common Esters Ester Species BoilingPoint (° C.) Ester Species Boiling Point (° C.) methyl-acetate 56.9ethyl-acetate 77.1 methyl-formate 32 ethyl-formate 54.0 methyl-lactate145 ethyl-lactate 151-155 dimethyl-malate 104-108 (1 mmHg)diethyl-malate 281.6  dimethyl-succinate 200 diethyl-succinate 217-218trimethyl-citrate    176 (16 mmHg) triethyl-citrate 235 (150 mmHg)

After recovering the esters in the remaining solution, the materials arein a readily usable form and one can either distill the ester mixture toseparate the different ester species and any remaining alcohol. Once theesters are recovered, one can use the monoesters as precursors forconversion into chelating agents, and the diesters as solvents.

An advantage of recovering the carboxylic acids from fermentation in theform of their corresponding esters is that downstream processing of theesters is less energy intensive than the hydrogenation of the freeacids. Another advantage of the present esterification process is that,one will find the present process simpler and easier, as compared toother approaches, to refine carboxylic acids for C₄ chemical platformsfrom fermentation. It simplifies efforts to separate esters from theother insoluble materials, as well as minimizes the amount of salt thatone needs to separate. In an integrated process enables one to directlyesterify a combination of free acid and salts that is produced in alow-pH fermentation, in which the fermentation is operated at a pH ofless than the pKa of the carboxylic acids.

FIG. 2 shows a schematic diagram of a downstream processing thatincorporates an iteration of the present esterification process. Inparticular, FIG. 2 depicts an example of using succinic acid or anyother kind of carboxylic acid derived from a fermentation broth isextracted and reacted with an alcohol in the presence of excess CO₂ togenerate esters. According to this iteration of the process, glucose,corn steep liquor, or other sugars, and Mg(OH)₂/NaOH are introduced intoa fermentation reactor 21 and fermented 23 to produce carboxylic acids.A fermentation broth liquid 25 containing a mixture of carboxylic acids,salts (e.g., succinic acid and its sodium or magnesium salt), and otherby-products is filtered 27 to remove cell mass and other insolublematter 27 a. The fermentation is performed at a low pH value, in whichone starts at a higher pH (e.g., pH ˜7 or 8) and during the course ofthe fermentation, the pH value drops to about 2-3. One will produce amixture of salts and free acid present, for example, in a ratio range ofabout 9:1 w/w to 7:3 w/w of salt to acid. The fermentation broth isretrieved from a fermentation reactor at a pH value of less than the pKaof the carboxylic acids, (e.g., pH 5). Typically, the fermentation brothis at a pH value in a range between about 1.5 and about 4.5.

The broth extract is then dried 29 to a powder. When drying the mixedacid filtrate should remove as much water as possible. The drying stepcan be accomplished, for instance, by means of spray drying, drumdrying, or cryodesiccation. As with esterification in general,relatively low water content is desired, otherwise the reversiblereaction will tend to hydrolyze back to the dicarboxylic acid. In thepresent process, a maximum residual moisture content of about 5% byweight should be maintained. One would expect an increase in ester yieldof up to about 98 or 99% with samples that contain less than 3% wt. ofwater.

The dried powder (average moisture content between about 1 wt. % and 5wt. %, desirably ≤3 wt. %) is then reacted 31 with an alcohol 33 whichserves as an alkylating agent, in excess CO₂ at a temperature betweenabout 180° C. to about 250° C. for a duration of about 4 hours or moreto esterify the carboxylic acids. In this example, succinic acid isreacted in methanol and CO₂ to generate dimethyl succinate. Along withthe free carboxylic acid, any remaining free amino acids which were inthe fermentation broth are also esterified.

Once the carboxylic acid esters are generated and recovered from thefermentation broth they can be further processed as feed stock in eitherhydrogenation or hydrogenolysis systems. Hydrogenation or hydrogenolysiscan be conducted according to various different methods, systems, andtheir permutations, such as described in U.S. Pat. No. 7,498,450B2(relating to homogenous hydrogenation of dicarbocilic acids and/oranhydrides), U.S. Pat. No. 6,433,193B, or 5,969,164, (relating to aprocess for production of tetrahydrofuran and γ-butyrolactone byhydrogenation of maleic anhydride); U.S. Pat. No. 4,584,419A (relatingto process for the production of butane-1,4-diol involving thehydrogenation of a di(C₁ to C₃ alkyl) ester of a C₄ dicarboxylic acid);UK Patent Application No. GB2207914A (relating to a process forproduction of a mixture of butane 1,4-diol, γ-butyrolactone, andtetrahydrofuran from maleate and fumerate); International PatentApplication Nos. WO8800937A (relating to a process for the co-productionof butane-1,4-diol and γ-butyrolactone by means of hydrogenation ofdialkyl maleate) or WO 82/03854 (relating to a process forhydrogenolysis of a carboxylic acid ester), or an article by S.Varadarajan et al., “Catalytic Upgrading of Fermentation-Derived OrganicAcids,” BIOTECHNOL. PROG. 1999, 15, 845-854, the content of each of thepreceding disclosures is incorporated herein in its entirety byreference.

As the example illustrates in FIG. 2, when reacted with methanol inaccord with the reaction temperatures and pressure parameters definedabove, succinic acid esterified to produce dimethyl succinate (aspredominant product), NaHCO₃, MgCO₃/Mg(HCO₃)₂ and excess methanol 35.The dimethyl succinate and methanol 37 are separated from NaHCO₃ andMgCO₃ 39. The carbonates, unlike CaSO₄, can be recycled 41 back into thereactor 21, either for a continuous process or in a fresh batch process.The dimethyl succinate and methanol are further separated 43 from eachother with the methanol 33 being recycled 45. Subsequently, the dimethylsuccinate 47 is subjected to either hydrogenation or hydrogenolysis 49and transformed into a variety of biofuel products 51, including alkanesand/or alcohols, for instance: ethane, ethanol, propane, propanol,butane, or butanol.

Another advantage of the present process is that it can simplify thetransport and processing of crops for fermentation products. Forinstance, with a dried fermentation broth powder one is freed fromissues associated with working with wet or liquid stock. A driedfermentation broth powder can be more economically shipped to a locationdifferent from where the fermentation broth is made or obtained. Thiswill enable the reaction for ester synthesis to be performed at a remotelocation different from where the fermentation broth is obtained, andexpand the geography of where the final processing facilities can besituated.

Section II.—Examples A

Examples prepared according to the present esterification method areintegrated into a process for isolating free carboxylic acid from afermentation broth. The method involves generally the following steps:a) filtering a crude fermentation broth to remove cell mass and otherbiological debris from a fermentation broth; b) desiccating thefermentation broth; c) reacting the dried fermentation broth with anexcess of methanol (CH₃OH) or ethanol (C₂H₅OH) and carbon dioxide (CO₂)at a temperature about 150° C. up to the near critical or criticaltemperature and under near critical or critical pressure of the alcoholand/or CO₂ reagents, to produce a mixture of monoesters and diesters andcarbonate (NaHCO3/MgCO₃); d) filtering the reaction product to removeby-products; and e) purifying by distilling the esters.

The fermentation broth filtrate was dried to remove all or nearly all ofthe water to produce a powder of mixed organics. Using a spray dryer ordrum dryer, one aerosolizes the raw solution containing mixed carboxylicacids to desiccate into a powder. The desiccated powder is suspended inan alcohol solvent. The powder reacts with the alcohol according to theconditions described herein to esterify into either monoesters ordiesters.

Each of the following examples was performed according to the followinggeneral protocol, except for variations in reaction temperature,pressure, time, and/or acid species as indicated, mutatis mutandis. Tengrams of freeze-dried succinic acid fermentation broth (off-whitepowder) and 300 g of methanol were charged to a IL stainless steelvessel, jacketed, and fixed to a Parr reactor. While stirringmechanically at 1100 rpm, the internal headspace of the reactor vesselwas purged with N₂ and then pressurized initially to 400 psi with CO₂and heated to 180° C. for 5 hours. The internal pressure was observed tobe ˜1650 psi at 180° C. After the reaction time, the reactor body wascooled in a water bath until reaching room temperature and pressurereleased. The heterogeneous mixture was then filtered and solids weredried overnight under vacuum. Samples of the solid material and thesolution were analysis quantitatively using gas-chromatography/massspectrometry (GC/MS). The yield of dimethyl succinate was determined tobe 31.9% with more than 95% of the available magnesium succinateconsumed in the reaction. The remaining balance of product included thecorresponding monoesters as the greater part, and was in a range ofabout 60% to about 65%.

As the reactions depicted in the accompanying figures and tables show,modification and selection of certain temperature and pressureparameters causes reactions to yield preferentially more of the diestercompounds. In certain examples of the present process, theesterification reactions yielded more than 50%, typically more than 70%or 80% di-alkyl succinate or malate. As stated before, the unreactedmaterials and the undesired products are recycled into the fermentationreactor. Subsequent separation of the mono-esters and di-esters wasachieved by crystallization.

FIG. 3 shows a series of esterification reactions which summarizeCO₂-assisted esterification of free succinic acid in various alcohols.FIG. 3A shows succinic acid reacted with methanol in 400 psi CO₂ gas, at150° C. for 5 hours, which achieved a yield of about 37% dimethylsuccinate. When the operational temperature was increased to 180° C. inthe reaction of FIG. 3B and all other parameters kept the same as inFIG. 3A, the amount of dimethyl succinate yield increases more thantwo-fold to about 81.2%.

FIG. 3C represents free succinic acid reaction at 180° C. under presentoperational conditions in ethanol, which generates diethyl succinate ingood yield of about 60.8%. In FIG. 3D, free succinic acid was reacted at180° C. under operational conditions in n-butanol, which generatesdibutyl succinate at about 52.2% yield. These examples demonstrate theversatility of the present esterification reaction in view of differentkinds of alcohols.

FIG. 4 shows examples of CO₂-assisted esterification of other kinds ofcarboxylic polyacids. In FIGS. 4A and 4B, succinic acid was substitutedrespectively with citric acid, a tricarboxylic acid, and malic acid. Theyield of triethylcitrate was reasonable at about 20.1%, demonstratingthat the CO₂-assisted protocol can be applied to tricarboxylic acids.The yield of the dimethyl analogue of malic acid was good at about84.3%. Hence, the new method of esterification is feasible for generaluse with other acids.

Table 4 summarizes results of several reactions that were performedaccording to the esterification method of the present disclosure asdepicted in FIGS. 5, 6, and 7. Each set of examples is arranged in termsof a variation of an operational condition under which the reaction wasperformed: A) temperature, B) pressure, and C) reaction time. In each ofthe examples, succinic acid from a fermentation broth is used as thesubstrate. The filtered clarified broth containing free acid and saltsare dried and later reacted with methanol and CO₂ in solution. (As thereactions are heated, the actual operational temperatures and pressureswithin the reactor vessel will exceed the initial temperatures andpressures provided herein.)

In the three examples of Set A, we carried out the reaction for 5 hoursat an initial CO₂ pressure of 400 psi, under different temperatures: Ex.A-1 at 180° C., Ex. A-2 at 210° C., and Ex. A-3 at 230° C. The percentconversion of acid to its corresponding diester increased with higheroperational temperature. FIG. 6 shows the effect of varying temperaturein a series of esterification reactions of succinic acid and its salt.In FIG. 5A, the esterification of succinic acid is performed at atemperature of about 180° C., over a period of 5 hours. The reactionproduced about 13.9% yield of dimethyl succinate. FIG. 5B shows the samereaction as in FIG. 5A, when the reaction time held constant, but withthe temperature raised to about 210° C., which yields about 42.9%. FIG.5C shows a reaction at 230° C. and yields about 72.4%. This suggeststhat as the temperature increases, the reaction kinetics drives toward amore complete reaction of the acid and alkylating agent, and a greateryield of the dialkyl-ester. Reactions performed at or near criticaltemperature and/or pressure conditions can produce at least 95%, likely≥97% or 98%, conversion.

In Set B and FIG. 6, we performed the esterification reaction for 5hours at an initial temperature of 180° C., and varied the initial CO₂gas pressures: Ex. B-1 at 400 psi, Ex. B-2 at 500 psi, and Ex. B-3 at600 psi. The percent conversion of acid to its corresponding diester wasmoderate, and the amount yield did not show significant differencestatistically. The initial CO₂ gas pressure in the reactor did not exertmuch effect in conversion of the acid to its diester, but theoperational pressures in the reactor during the reaction suggest aneffect on yields.

In Set C and FIG. 7, we performed the esterification reaction at aconstant pressure and temperature but varied the duration of thereaction. Ex. C-1 at 5 hours, Ex. C-2 at 2 hours, and Ex. C-3 at 0.5hours. The examples shown in FIG. 8 suggest that a greater amount ofdiester was converted from the acid with increased reaction time.

FIG. 8 shows a first set of CO₂-assisted esterification reactions usinga concentration of succinate salts of about 4% w/w, which are presentedas Examples 1-3 in Table 5. In Examples 1 and 2, succinic acid and itsmagnesium (Mg²⁺) salt was reacted in methanol and ethanol at 210° C. and180° C., respectively, for a reaction time of 5 hours. The reactionsproduced about 33% dimethyl succinate and about 1% diethyl succinate,respectively. Methanol exhibits a greater capacity to dissolve thesuccinate salt than ethanol. Magnesium succinate exhibits a reasonablelevel of solubility in methanol, while it exhibits limited solubility inethanol, even at high temperatures. Hence, the yield of diethylsuccinatewas negligible. Example 3 shows a reaction using calcium (Ca²⁺)succinate, at 180° C., over 5 hours. The reaction yields only about1.33% of the corresponding dimethylsuccinate. Relatively low conversionrates in Examples 2 and 3, also highlights the solubility differencebetween corresponding alkali earth salts. The calcium succinate salt isinsoluble in methanol, even at high temperatures. The methanol to saltmolar ratio used in the CO₂ experiments was approximately 110:1 formethanol to magnesium succinate. Likewise, the ratio was about 100:1 formethanol to the other carboxylic acids.

TABLE 4 Variations in Reaction Conditions Initial CO₂ % Conversion toExample Substrate Alcohol Reaction Time (h) Temperature (° C.) pressure(psi) Diester A 1 Succinic acid Methanol 5 180 400 13.9 fermentationbroth, Mg²⁺ salt 2 Succinic acid Methanol 5 210 400 49.2 Temperaturefermentation broth, Variation Mg²⁺ salt 3 Succinic acid Methanol 5 230400 72.4 fermentation broth, Mg²⁺ salt B 1 Succinic acid Methanol 5 180400 13.9 fermentation broth, Mg²⁺ salt 2 Succinic acid Methanol 5 180500 11.4 Pressure fermentation broth, Variation Mg²⁺ salt 3 Succinicacid Methanol 5 180 600 9.6 fermentation broth, Mg²⁺ salt C 1 Succinicacid Methanol 5 180 400 13.9 fermentation broth, Mg²⁺ salt 2 Succinicacid Methanol 2 180 400 5.4 Reaction Time fermentation broth, VariationMg²⁺ salt 3 Succinic acid Methanol 0.5 180 400 ND fermentation broth,Mg²⁺ salt

TABLE 5 Reaction Initial CO₂ % Conversion to Example Substrate AlcoholTime (h) Temperature (° C.) pressure (psi) Diester Note 1 Succinic acid,Mg²⁺ salt Methanol 5 210 400 33.4 Control 2 Succinic acid, Mg²⁺ saltEthanol 5 180 400 1.0 Limited solubility 3 Succinic acid, Ca²⁺ saltMethanol 5 180 400 1.3 Limited solubility 4 Succinic acid Methanol 5 150400 37.0 5 Succinic acid Methanol 5 180 400 81.2 6 Succinic acid Ethanol5 180 400 60.8 7 Succinic acid 1-Butanol 5 180 400 52.2 8 Citric acidMethanol 5 180 400 20.1 9 Malic acid Methanol 5 180 400 86.3

Table 5 lists results from other examples of esterification reactionsaccording to the present method. Examples 1, 2 and 3 demonstrate theimportance of substrate solubility of succinic acid as compared to thesalts of succinate. Examples 4-7 is a second set of reactions in whichfree succinic acid was reacted in methanol, ethanol, and 1-butanol insimilar fashion. Examples 8 and 9 show that reactions with othercarboxylic acids, such as citric acid and malic acid can achieverelatively good yields of about 20% and 86%, respectively.

Free succinic acid reacts readily with the alcohols, since it iscompletely soluble in methanol, ethanol, butanol, and other alcohols upto and including octanol (C-8 alcohol). In Examples 6 and 7, succinicacid reacted in ethanol and 1-butanol, yields 60.8% and 52.2%conversion, respectively.

The solubility of carboxylic salts in a particular solvent can have aninfluence on the esterification process. The greater solubility offree-acid permits a greater reactivity than the carboxylate salt, whichlacks an acid functionality. Accordingly, the yields of thecorresponding esters tend to be significantly greater than the controlsamples when comparing the two sets of reactions. The reactions ofExamples 4-7 yielded significantly greater amounts of correspondingdiesters than that of Examples 1-3. The carboxylic acid itself may besufficient to catalyze the esterification reaction under the presentoperational temperature and pressure conditions. One can adjust thesubstrate solubility for successful esterification according to thepresent method.

B Example: Hydrogenation

Using a process like one of those described in the references citedabove, one can perform direct hydrogenation of the carboxylic acidesters collected from the esterification process described above. Forexamples, one can use metallic copper catalysts for the hydrogenation ofdialkyl succinate esters to BDO, GBL, and THF. The following describesan illustration of the hydrogenation process.

The copper catalysts were prepared by wet impregnation of copper saltsonto the following supports (all −16+30 mesh): silica-alumina (93%silica, 7% alumina Sigma Chemicals) and two chromatographic silicas(Phase Separations, Inc.) (XOA-400 and XOB-030). Ten milliliters ofsolution containing copper nitrate to produce the desired loading wasadded to 10 g of support. The slurry was stirred at room temperature for2 hrs. and then dried for 2 hrs. under vacuum at 60° C.-70° C. The driedsolid was then calcined in air in a furnace at 500° C. for 11-12 h togive copper oxides. The catalyst material was then loaded into thereactor and reduced in-situ in pure hydrogen at 200° C. and 200 psig for3 hrs. Catalyst supports were characterized before and after reaction bynitrogen BET surface area and mercury porosimetry. Support aciditymeasurements were made using n-butylamine as a titrant in dry benzenewith p-dimethylaminazobenzene (pK_(a)=3.9) as the indicator.

A cone closure reactor (Autoclave Engineers, Inc.) made of 316 StainlessSteel, 167 mm long×5 mm inner diameter (i.d.), was charged with one gramof catalyst supported on a quartz frit. The reactor was surrounded by aclamshell furnace controlled by an Omega series CN-2010 programmabletemperature controller. Dimethyl succinate was fed to the top of thereactor as 30 wt % solution in methanol using a Biorad HPLC pump.Hydrogen gas was also fed to the top of the reactor from a standard tankand high-pressure regulator; a rotameter was used to monitor gas flowrate at the reactor outlet. The liquid feed rate was fixed at 0.05mL/min, and the hydrogen rate was set at 400 mL of STP/min to give aweight hourly space velocity (WHSV) of 0.9 g of DMS/g of cat/h and aH₂/succinate ratio of 200:1.

Condensable products were collected in single-ended 10 mL Whitey samplecylinders immersed in ice baths. Three-way valves were used to divertreaction products to either of two sample cylinders: during reaction,condensable products were collected in the traps for a timed period,after which the trap was removed and weighed and the contents wereremoved for analysis. Gas exiting the collection cylinders passedthrough a rupture disk assembly and was stepped down to atmosphericpressure using a back-pressure regulator. Gas products were collected ingas bags for analysis using gas chromatography to quantifynon-condensable product formation.

Condensable products were weighed following collection and analyzed in aVarian 3300 gas chromatograph equipped with a flame ionization detectorand a Supelco SPB-1 wide-bore (0.5 mm) capillary column (50-200° C. @12° C./min, hold @ 200° C.). Methyl lactate was used as an internalstandard to facilitate the calculation of product concentrations.

Example: Hydrogenolysis

The esters resulting from the fermentation extraction described above isthen hydrogenolyzed over a catalyst (e.g., reduced CuO/ZnO), whichshould obtain high conversions (>98%) and selectivities (e.g.,International Patent Application No. WO 82/03854).

Alternatively, one can proceed according to a process such as describedin U.S. Pat. No. 4,584,419. A stainless steel with an oil jacketmaintained at 231° C. was used for this reaction. Hydrogen wasintroduced by way of a pressure regulator and flow controller (notshown) through line to the bottom end of a vaporizer containing a numberof steel balls. Ester was metered as a liquid to vaporiser through aline. The resulting vaporous mixture of ester and hydrogen was passedthrough preheating coil to reactor. This contained a layer of glassballs, on which rested the catalyst bed. The remainder of the reactorwas filled with glass balls and the upper end of the reactor was fittedwith an exit tube which led to a condenser (not shown) and then to apressure let-down valve. The exit gas flow rate was measured downstreamfrom the condenser using a wet gas meter.

A charge of 30 ml of a granulated copper chromite catalyst was placed inthe reactor which was then purged with nitrogen at 42 bar. The oil bathwas raised to a temperature of 231.degree. C. A 2% H.sub.2 in N.sub.2gaseous mixture at 42 bar was then passed over the catalyst for 8 hours,followed by 10% H.sub.2 in N.sub.2 (still at 42 bar) for a further 16hours, and then by pure H.sub.2 (also at 42 bar) for an additional 12hours.

Diethyl succinate was then introduced into the vaporizer correspondingto a liquid hourly space velocity of 0.2/hr. The hydrogen gas:estermolar ratio in the vaporous mixture was 313:1. The temperature of thesand bath was maintained at 231° C. The condensate was analyzed by gaschromatography using a 1.82 meter long stainless steel column with aninternal diameter of 3.18 mm containing 10% diethylene glycol succinateon Chromosorb PAW, a helium gas flow rate of 30 ml/minute and a flameionisation detector. The instrument was fitted with a chart recorderhaving a peak integrator and was calibrated using a mixture of diethylmaleate, dialkyl succinate, butyrolactone, butane-1,4-diol,tetrahydrofuran and water of known composition. The exit gas was alsosampled and analyzed by gas chromatography using the same technique. Theidentity of the peaks was confirmed by comparison of the retention timesobserved with those of authentic specimens of the materials in questionand by mass spectroscopy. The following compounds were detected in thereaction mixture: diethyl succinate, butyrolactone, butane-1,4-diol,tetrahydrofuran and water. Trace amounts of minor byproducts, including2-ethoxytetrahydrofuran and 2-ethoxybutane-1,4-diol were also detectedin the reaction mixture. From the results obtained it appeared thatdiethyl succinate had been smoothly converted to products with aselectivity to tetrahydrofuran of 52.2 mol %, a selectivity to n-butanolof 11.6 mol %, a selectivity to gamma-butyrolactone of 26.1 mol %, and aselectivity to butane-1,4-diol of 10.1 mol %, the balance being minorbyproducts.

The present invention has been described in general and in detail by wayof examples. Persons of skill in the art understand that the inventionis not limited necessarily to the embodiments specifically disclosed,but that modifications and variations may be made without departing fromthe scope of the invention as defined by the following claims or theirequivalents, including other equivalent components presently known, orto be developed, which may be used within the scope of the presentinvention. Therefore, unless changes otherwise depart from the scope ofthe invention, the changes should be construed as being included herein.

The invention claimed is:
 1. A process for producing a biofuelcomprising: a) using a fermentation broth containing at least one freecarboxylic acid; b) synthesizing an ester by reacting said freecarboxylic acid with an alcohol solvent under a CO₂ atmosphere insubstantial absence of any other acid catalyst at either a reactiontemperature or pressure or both that is at a supercritical, critical ornear critical condition for at least one of the alcohol or CO₂; and c)subjecting said ester to either hydrogenolysis or hydrogenation to forma biofuel.
 2. The process according to claim 1, further comprisingrecovering said biofuel.
 3. The process according to claim 1, whereinsaid biofuel is an alkane or an alcohol.
 4. The process according toclaim 3, wherein said biofuel includes at least one of the following:ethane, ethanol, propane, propanol, butane, 1-butanol, octane, andoctanol.
 5. The process according to claim 1, further comprising dryingsaid fermentation broth into a powder before ester synthesis.
 6. Theprocess according to claim 1, further comprising concentrating saidester before either hydrogenolysis or hydrogenation.
 7. The processaccording to claim 1, further comprising purifying said ester to atleast 90% purity.
 8. The process according to claim 1, wherein saidfermentation broth is at a pH in a range between about 1.5 and about4.5.
 9. The process according to claim 1, wherein said alcohol solventhas an R-group of C₁-C₂₀, and is at least one of a saturated,unsaturated, cyclic, or aromatic species.
 10. The process according toclaim 1, wherein said carboxylic acid is selected from the groupconsisting of: formic acid, acetic acid, propionic acid, lactic acid,butyric acid, isobutyric acid, valeric acid, hexanoic acid, heptanoicacid, decanoic acid, lauric acid, myristic acid, and C₁₅-C₁₈ fattyacids, fumaric acid, itaconic acid, malic acid, succinic acid, maleicacid, malonic acid, glutaric acid, glucaric acid, oxalic acid, adipicacid, pimelic acid, suberic acid, azelaic acid, sebacic acid,dodecanedioic acid, glutaconic acid, ortho-phthalic acid, isophthalicacid, terephthalic acid, citric acid, isocitric acid, aconitic acid,tricarballylic acid, and trimesic acid.
 11. The process according toclaim 1, wherein said carboxylic acid is a polycarboxylic acid.
 12. Theprocess according to claim 11, wherein said carboxylic acid is adicarboxylic or a tricarboxylic acid.
 13. The process according to claim11, wherein when said carboxylic acid is a polycarboxylic acid, saidester is at least a diester.
 14. The process according to claim 11,wherein said polycarboxylic acid is converted to a corresponding esterat a rate of at least 50% of said polycarboxylic acid.
 15. The processaccording to claim 1, wherein said reaction temperature is between about150° C. and about 250° C., and said pressure is in a range between about400 psi and 3,000 psi.
 16. The process according to claim 1, whereinsaid free carboxylic acids are not subject to activation with a halideto form an acyl halide.